Flow-dependent verification of the ECMWF ensemble over the Euro-Atlantic sector

In this article the concept of weather regimes is used to assess the flow-dependent skill of the European Centre for Medium-range Weather Forecast (ECMWF) ensemble predictions at the late medium-range. The flow patterns leading to more or less accurate predictions are identified and the variations of skill in these situations are quantified. The focus is on the Euro-Atlantic sector during the extended winter period when the atmospheric regime structure is most pronounced. Verification results show that, in the medium range, forecasts initiated in the negative phase of North Atlantic Oscillation (NAO-) are the most skillful. For these forecasts the ensemble spread over Europe is lower than average, showing that the ensemble spread provides useful information about the error of the ensemble-mean forecast. The performance of the ensemble is further assessed by stratifying the cases according to their initial conditions, as well as by their accuracy at forecast day 10. Results indicate that the least skillful predictions are mainly associated with missing the transitions to a blocking regime circulation. Forecasts also underestimate the blocking persistence, whereas they overestimate the persistence of zonal flows. Transitions to a positive phase of the NAO are also overrepresented.